1. Field of the Invention
The present invention relates to a modulator for use in television signal transmitters and the like.
2. Description of the Prior Art
A television signal transmitter for use in CATV systems and the like uses a modulator for modulating a carrier wave with video signals to obtain intermediate frequency signals of the PAL specification (European specification) in the 30 MHz band or the NTSC specification (U.S. specification) in the 40 MHz band.
FIG. 4 illustrates the configuration of such a modulator according to the prior art. Video signals Vd serving as modulation signals are inputted to the inverted input end (−) of an operational amplifier 31, and to its non-inverted input end (+) is applied a reference voltage Er.
As a video mixer 32 consists of a double-balance mixer having a first differential amplifier 32a and a second differential amplifier 32b each having two transistors, emitters of a first transistor TR1 and a second transistor TR2 in the first differential amplifier 32a are connected to each other, and those of a third transistor TR3 and a fourth transistor TR4 in the second differential amplifier 32b are also connected to each other. An output end of the operational amplifier 31 is connected to the emitters of the first and second transistors TR1 and TR2.
Bases of the first transistor TR1 and the fourth transistor TR4 are connected to each other, and those of the second transistor TR2 and the third transistor TR3 are also connected to each other. Further, collectors of the first transistor TR1 and the third transistor TR3 are connected to each other, and those of the second transistor TR2 and the fourth transistor TR4 are also connected to each other.
The emitters of the first and second transistors TR1 and TR2 are grounded via a resistor 33, and those of the third and fourth transistors TR3 and TR4 are grounded via a resistor 34.
Further, the collectors of the first and third transistors TR1 and TR3 and those of the second and fourth transistors TR2 and TR4 are connected to a terminal on one side of a balanced-to-unbalanced transformer 35, whose terminal on the other side is connected to a power supply terminal 37 via a choke inductor 36, and the other terminal on the other side is grounded on a high frequency basis by a capacitor 38.
Between the connection point of the balanced-to-unbalanced transformer 35 and the choke inductor 36 and an output end 39 is connected a resistance attenuator 40 in a π pattern.
A carrier wave signal Cw is entered into a balanced amplifier 43 via a resistance attenuator 41 and a balanced-to-unbalanced transformer 42 configured in a π pattern, amplified and entered in a balanced way between the bases of the first and fourth transistors TR1 and TR4 and those of the second and third transistors TR2 and TR3.
In the configuration described above, the video mixer 32 operates in an unbalanced input pattern with respect to video signals and in a balanced input pattern with respect to carrier wave signals. From the output end 39, a signal whose waveform is modulated as shown in FIG. 5A, i.e. a compound picture signal is supplied, whose modulation factor (M/C) is set by the reference voltage Er of the operational amplifier 31.
However, as the currents of the transistors in the double-balance mixer 32 vary with the ambient temperature, their gains also vary, and eventually the modulation factor varies, too.
If, for instance, a variation in temperature increases the transistor currents and their gains also rise, the amplitude of the compound video signal expands from M to M′ as its waveform shown in FIG. 5B indicates. On the other hand, as the reference voltage Er is constant at the minimum level E of the picture signal, the modulation factor M′/C′ varies in an increasing direction from before the temperature variation.